On Thu, May 8, 2014 at 1:49 PM, Jan Steinman via EV <[email protected]> wrote: >> From: David Nelson via EV <[email protected]> >> >> No. Don't confuse charge capacity with energy capacity. None of the >> charge capacity gets turned into heat, only some of the energy >> capacity gets turned into heat. > > Other than for pedantic interest, what's the difference? If some of the > battery's capacity gets turned into heat, it still isn't available to the > wheels, no? > > Perhaps you can give us a mini-tutorial on the difference between SOC and > SOE. Because this EE is confused. >
State of charge is exactly what it says. Charge does not equal energy. Just like mass doesn't equal potential energy. There is more to energy than just charge. You have to have a voltage (or height in the case of gravitational potential energy) to determine energy. With any "rocking horse" type battery like LiFePO4 the Ah capacity is merely a way to count the number of electrons which traverse through the external circuit. An electron isn't energy. It can have energy but isn't energy in and of itself. Try to answer this question and you should get my point. I have a 100Ah battery, how much energy does it hold? You can't answer it, can you? With no other information don't know. You have to know something more about the battery so that you can at least extract the nominal voltage of the battery to then get a close approximation of the energy it stores. State of energy, on the other hand, is about the actual energy stored in the battery. Like Lee pointed out, some may want to track how much energy is in their battery so they know how far they can go before they run out and others would rather track the state of charge of our batteries to keep it between our chosen ranges. With LiFePO4 these track rather closely within a given set of conditions. Here is, however, the problem I have with tracking energy rather than charge with my battery pack. I know from experimentation that under warm conditions, meaning above 20°C ambient, that the nominal voltage for my pack is 64V (3.2V/cell). This means my total 100% energy storage capacity of my pack is 200Ah*64V=12.8kWh. When my pack is cold, especially below 10°C, the nominal voltage is somewhat lower. I don't have a hard number for it but it is in the 3.0-3.1V/cell range. If we use 60V nominal for the pack now I only have 200Ah*60V=12.0kWh of energy storage. What do I use as the energy capacity for my pack now? What stayed the same in both of those calculations? The number of electrons available to run through the external circuit. When you count coulombs none of the other stuff matters when determining what state the battery pack is in. If I want to stop when it is at 80%SOC and charge I can hit it every time regardless of the other conditions but if I want to stop at 80%SOE then I have to constantly try to adjust how much energy is really available. >> With LiFePO4 >> cells, however, the warmer they are the more energy efficient they >> become so it actually doesn't affect range as much as you might first >> think. > > I'm getting more and more confuseder by the minute! > > You just said the batteries had 100% charge/discharge efficient. Now you say > heating from internal resistance makes them even more efficient. > > When they heat up to 110% efficiency, can't we just eliminate charging > altogether? :-) > You missed a critical word: energy. I said it is more ENERGY efficient. I didn't say anything about the coulombic efficiency. When I have had a nearly 70 mile continuous run pulling 135A nearly continuously from my pack I have left home with a case temperature of 7°C and arrived with a case temperature at 22°C. The interesting part was that the loaded voltage stayed nearly constant even though the batteries went from 100%SOC to 13%SOC. On another slightly shorter run the battery temp went from 6°C to 18°C with the SOC going from 100% to 27%, again with the loaded voltage staying relatively constant. >> Also, to assume voltage stays fairly constant with changing current is >> way off the mark of what happens. > > What I meant by "fairly constant" is that the terminal voltage changes by > millivolts as the current through the internal resistance goes from 0 to a > couple hundred amps. So yea, that change in voltage times the current times > time is the energy "lost" to heat. > You must have some awesome batteries. Mine sag more than by just a few millivolts. With cold batteries I've seen a fully charged pack drop from 67.5V to under 57V under a 200A load. That is over 0.5V drop per cell. Is that what you mean or are you talking smaller drops in voltage? Of course 12.8kWh to 12.0kWh is only a loss of about 6% so maybe that is what you figure is small. Hope that makes sense. -- David D. Nelson http://evalbum.com/1328 http://www.levforum.com _______________________________________________ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org For EV drag racing discussion, please use NEDRA (http://groups.yahoo.com/group/NEDRA)
